1. Relevant Field
The present disclosure relates to radio-frequency identification. More particularly, the present disclosure relates to a charge pump stage of a radio-frequency identification transponder.
2. Description of Related Art
Radio Frequency Identification (RFID) systems are widely used for identifying and tracking items, inventory control, supply chain management, anti-theft of merchandise in stores, and other applications. A typical RFID system consists of a plurality of transponders (hereinafter referred to as “RFID tags” or simply as “tags”) and one or more transceivers (hereinafter referred to as “RFID readers” or simply as “readers”). A reader interrogates one or more of the tags over a wireless forward link such as an UHF signal. The tags respond to the reader's interrogations by transmitting tag information back to the reader over a wireless return link.
A tag is usually embodied as a semiconductor microchip having a memory for storing the tag's ID number and, in some applications, information concerning the item to which the tag is associated. Further, tags are either “passive” or “active”, depending on how they are powered. An active tag contains its own on-board power source, i.e. a battery, which the tag uses to process received signals and to transmit tag information back to a reader. A passive tag does not have its own on-board power source. Rather, it derives the power it needs by extracting energy from the RF carrier signals broadcasted by the reader. The passive tag transmits information to the reader using a process known as load modulation for inductive systems or backscattering for wave propagation systems. A battery assisted tag contains its own on-board power source, i.e. a battery, like an active tag, which the tag uses to process received signals and to power the digital circuit and the memory. However, the battery assisted tag transmits information back to the reader using the same principles as passive tags such as by modulating a backscattered signal. Passive tags have, in many applications, become more popular than active tags, since they are less expensive to manufacture, to maintain, and to operate.
Because passive tags do not have their own power sources, power must be derived from the RF interrogating signal. Typically, an RFID tag includes a loop antenna tuned to receive the RF interrogating signal. The loop antenna is electrically connected to a charge pump of the RFID tag. The RF interrogating signal induces an AC signal within the loop antenna that is provided to the charge pump. The charge pump rectifies and amplifies the voltage of the AC signal to charge a storage capacitor and/or to power digital circuitry. The storage capacitor also provides energy to modulate the RF interrogation signal.
The RFID reader typically generates an RF interrogation signal that is a continuous RF carrier sine wave. A shunting transistor across the loop antenna can be employed to modulate the amplitude of the RF interrogation signal. This modulation of the RF interrogation signal occurs because the absorption of the RF interrogation signal by the loop antenna varies as the shunting transistor is switched on and off. The RFID reader can sense this modulation. In other words, the RFID reader demodulates the modulated backscattered signal to receive the information from the RFID tag.